1. Field of the Invention
The present invention relates to improved oil based well bore fluids known in the oil service industry as drilling fluids, and, in particular, to oil based invert emulsion types of drilling fluids in which water is dispersed in an oil-based medium. The invention is particularly directed to providing enhanced viscosity and anti-settling properties to such fluids over the wide temperature ranges found in more recent drilling operations; that is, the ability of the fluids to possess the proper viscosity profile, and to retain in suspension in their structure and to convey along with the fluid, a variety of types of solid particles, the most important of which are bore-hole cuttings. These properties are particularly valuable when non-vertical directional and deep water drilling is undertaken.
2. Description of the Prior Art
The oil industry has used "drilling muds" or drilling fluids since the beginning of United States oil well drilling operations in Pennsylvania, Texas and Oklahoma. These drilling fluids are pumped under pressure down through the string of drill pipe already in the ground, then through the center of the drilling bit, and then return up through the space between the outside of the drill pipes and the borehole wall finally being brought back up to the surface. Drilling base fluids, the liquid carriers of the system, are often comprised of oils (diesel, mineral and poly(alpha-olefin)), propylene glycol, methyl glucoside, modified esters and ethers, water, and emulsions of oil and water of varying proportions.
A drilling fluid must accomplish a number of interrelated functions for it to satisfy the minimum requirements for a commercial drilling fluid. These functions can be grouped as follows:
(1) The fluid must constantly lubricate the drill bit so as to promote drilling efficiency and retard bit wear,
(2) The fluid must have a proper thickness or viscosity to meet the many different criteria required by the drill owner/operator,
(3) The fluid must provide filtration control,
(4) The fluid must suspend and transport solid particles to the surface for screening out and disposal, and
(5) The fluid must keep suspended solid particles and weighting agents (to increase specific gravity of the mud; generally barytes; a barium sulfate ore, ground to a fine particle size), when drilling is interrupted.
The above functions must be satisfactorily provided throughout the time the fluid is in the entire length of the drill hole. Since the drill hole can be as much as tens of thousands of feet long, varying and extreme temperatures are encountered, which temperature changes effect the fluid's physical properties and performance.
The interrelatedness of the above functions can be seen by the fact that the unwanted materials to be removed at the surface can include not only "cuttings" from the material through which the bit is passing, but also pieces of the drill bit itself, barytes or other weighing materials, and substances such as gellants, dissolved gases, and salts created when other fluid constituents become "spent" under the high temperatures encountered in the bottom of deep wells. Sometimes various constituents fuse into agglomerated particles using present additives if low temperatures are encountered in the "trip" back to the surface.
Finally, it should be noted that a drilling fluid must perform its various functions not only when the drill bit is actively encountering the bottom of the borehole, but also at all times and at all locations in the drill stem. In particular, cuttings must be held in suspension through their long journey back to the surface through regions of quite different and varying temperatures compared to that found in the hole at depth.
A drilling fluid is typically a thixotropic system; that is, (1) it exhibits low viscosity when sheared, such as on agitation or circulation (as by pumping or otherwise) but, (2) when such shearing action is halted, the fluid thickens to hold cuttings in place; the fluid must become thick relatively rapidly, reaching a sufficient gel strength before suspended materials fall any significant distance--and (3) this behavior must be totally reversible at all temperatures encountered. In addition, even when a free-flowing liquid, the fluid must retain a sufficiently high viscosity to carry all unwanted particulate matter from the bottom of the hole to the surface. To maintain these functions under the widely varying temperatures encountered in deep water drilling has proved extremely difficult with the use of commercial rheological drilling fluid additives presently available on the market.
One of the principal problems facing "mud chemistry" scientists is the production of thickening agents, thixotropes and drilling fluids having satisfactory dispersibility, with the necessary subsidiary thixotropic properties discussed above, while at the same time possessing critically important antisettling properties over a wide range of temperatures. While the compositions of these various fluids is considered a "black art" to many, in reality, fluids and their additives involve highly complex chemical, physical and rheological analysis using state-of-the-art scientific apparatus and intricate mathematical calculations and modeling.
A different measure of control during drilling occurs because of wide ranges of a) encountered temperature (from as low as below 5.degree. C. to as high as 200.degree. C.), b) time durations, c) pressures (from only a few bars to those exerted by a column of fluid that can extend for thousands of feet) and d) drilling directions (from vertical to horizontal).
Accordingly, a search has been going on for many years for an improved additive for modifying and controlling the suspension properties of drilling fluids that would be efficient, easily handled, and readily dispersible in a broad range of drilling muds, and be usable under a broad range of temperature and pressure conditions.
Drilling Mud Circulation
As was noted above, drilling fluid is pumped under pressure down through the string of drill pipe, through the center of the drilling bit, then through the annulus between the outside of the drill stem and the borehole wall, back up to the surface. This circulation constantly removes cuttings from the instantaneous bottom of the hole, and lifts them the entire distance from this bottom to the surface for disposal. Such a distance today can be in the thousands or tens of thousands of feet and involve quite remarkable changes of temperature.
Further, it is desirable for the drilling fluid to possess less dynamic anti-settling properties when being circulated down the drill pipe and out the bit, and to have higher viscosity and anti-settling properties while rising through the annulus. Unless the fluid removes cuttings from beneath the bit before the next bit tooth arrives, the cuttings will be reground into a finer particle size, and made more difficult to remove by screening and also materially slow down the rate, since the same material is being reground over and over again. The presence of unremoved cuttings in the fluid will decrease drilling penetration rates, with resultant increase in the overall costs of drilling the well.
Once in the annulus, the cuttings which are generally denser than the drilling mud itself, tend to settle downward under the influence of gravity. The upward velocity of the drilling fluid in the annulus must be higher than the settling rate, so as to bring the cuttings to the top of the hole. All of the above properties must largely be independent of temperature.
Off Shore Deep Water Drilling and Temperature Sensitivity
In modern times, hydrocarbon drilling for exploratory and production wells has increasingly been done from platforms located in water settings, often called off-shore drilling. Such fresh and salt water drilling employ floating barges and rigs fixed in some fashion to the submerged surface of the earth.
Economic and technical advances have recently pushed these drilling operations into deeper waters. Although advances in equipment and engineering have yielded technology capable of drilling in water depths up to 10,000 feet or more, advances required in drilling fluid technology have lagged.
A major problem with oil based drilling fluids in deepwater drilling is rheological additive temperature sensitivity over the temperature range encountered. During circulation, the drilling fluid typically reaches bottom hole temperatures of about 60.degree. C. to 80.degree. C. followed by cooling to lower than 5.degree. C. in the riser during its travel upward (due to the inherent low temperature of sea water far below the ocean surface). For successful deepwater drilling, the mud needs to suspend the solids and remain pumpable with proper viscosity over these wide temperature ranges.
Drilling fluids composed of conventional organophilic clay rheological additives particularly suffer considerable viscosity build as the drilling fluid is cooled from a temperature of 60.degree. C. to 5.degree. C., for example. As a result of this viscosity increase, the drilling fluid, when it reaches low temperatures, is more difficult to pump, the equivalent circulating density (ECD) is increased and increased drilling fluid losses to the formation (lost circulation) frequently occur.
The invention discloses new oil based drilling fluids, particularly oil invert drilling muds, which are distinguished by improved rheological properties, high ecological acceptability, and at the same time good storage and application properties. One important area of application for the new drilling fluid systems is in off-shore wells, the aim of the invention being particularly to make available industrially usable drilling fluids with enhanced properties over a large temperature range. The use of the new drilling fluid systems has particular significance in the marine environment, but is not limited to this field. The new mud system also can be put to use in land-based drilling operations as described below.
Directional Drilling
The requirements for drilling fluids with enhanced temperature properties also has become more complex over the past decade as a result of changes in directional drilling technology, in which a well is drilled at an angle other than vertical. Such wells are also known as deviated wells.
Methods for deviating wells have changed greatly over recent years with the production of more powerful and reliable downhole motors, and the invention of more accurate techniques utilizing wireline techniques as well as the highly computerized downhole, sensing and micro reduction equipment, including improvements in sounding apparatus and microwave transmission. These techniques permit the instantaneous obtaining of data relating to down-hole conditions without the need to remove the drill string.
The advantages of directional drilling are that it allows (1) the tapping of fields which cannot effectively be reached by vertical drilling; (2) permits the use of more economical land-based equipment to explore the immediate off-shore environment; and (3) allows the drilling of multiple wells up to several miles from one another, sharing the cost of a single platform. In certain formations, increased production can be achieved by deviating the well off-verticle so as to facilitate perforation and development of a narrow producing zone, or redevelopment of a depleted formation.
Use of a downhole motor allows the hole to be deviated by the introduction of a fixed offset or bend just above the drill bit. This offset or bend can be oriented by modern MWD systems which are capable of reporting accurately the current bit and toolface hole angle and azimuth (i.e. the orientation with respect to the upper portion of the hole). It is accordingly possible to rotate the drill string until the toolface has achieved the desired direction of deviation, and then to fix the drill string in place and commence the deviation by starting the motor to extend the hole in the desired deviated direction.
There are, however, a number of inherent problems in this approach to directional drilling, which affect the requirements of a drilling mud; namely:
As in deep water drilling, increased ranges of temperatures are encountered. PA1 The annulus carrying the mud to the surface is no longer vertical and extends to far greater distances versus vertical wells. PA1 Gravity on a horizontal hole pulls cuttings, weighting material and particulate matter, not controlled by the drilling fluid, to the bottom side of the bore (not the bottom of the hole as in traditional drilling) and results in drag on the bore wall. PA1 The amount of drilling mud required is increased since the distances are greater, and the time required for the mud to reach the earth's surface also increases. PA1 Curves and kinks in the hole's direction can accumulate cuttings and additives. PA1 a) preparing an oil based drilling fluid base composition; and PA1 b) incorporating into such an oil based drilling fluid base composition; PA1 a) an oil based drilling fluid base composition; PA1 b) one or more organoclays made as described hereafter and PA1 c) one or more castor wax type rheological additives. PA1 (a) a smectite-type clay having a cation exchange capacity of at least 50 milliequivalents per 100 grams of pure clay; and PA1 (b) one or more quaternary ammonium compounds in an amount of from about 40% to about 200% of the cation exchange capacity of the smectite-type clay.
In order to obviate or mitigate these problems, which can cost oil and gas companies millions of dollars per hole, it is an object of the invention to provide drilling fluids with rheological properties particularly appropriate for directional drilling apparatus in addition to the increased viscosity modification stability with temperature discussed above.
Prior Art
U.S. Pat. No. 5,021,170 describes a viscosifying gellant for oil-based well bore fluids comprising a mixture of an organoclay and a sulfonated, ethylene/propylene/5-phenyl-2-norbornene terpolymer. Japanese Patent Application No. 62-69957 describes a sag preventer for non-aqueous coating materials comprising a mixture of two different fatty acid amides wherein fatty acid amide (A) is obtained by reacting a mixture of at least one straight chain saturated fatty acid having 3-4 carbon atoms and 12-hydroxystearic acid (the molar ratio of the fatty acid and 12-hydroxystearic acid being 1:9-8:1) and ethylene diamine or hexamethylene diamine and fatty acid amide (B) is obtained by reacting a mixture of at least one straight chain saturated fatty acid having 6-22 carbon atoms and 12-hydroxystearic acid (the molar ratio of the fatty acid and 12-hydroxystearic acid being 0:1-8:2) and ethylene diamine or hexamethylene diamine, wherein the weight ratio of fatty acid amide (A) to fatty acid amide (B) is 100:00 20:80.
U. S. Pat. No. 3,252,820 describes a rheological composition containing a thixotrophic castor wax derived from glyceryl trihydroxystearate. Rheox, Inc., assignee hereof, offers for sale as a commercial product a rheological additive designated THIXCIN R which is based on castor wax as well as other products based on low hydroxyl value castor wax. Rheox also offers for sale products utilizing castor wax and extenders such as clay and aluminum silicate.
U.S. Pat. No. 4,631,136 describes plant or vegetable oil-based drilling fluids containing a minor amount of a viscosifier consisting of an amine-treated bentonite clay. European Patent No. 00583285 B1 teaches use of clays treated with ester quaternary compounds to improve their oil wettability and the use of such clays to prepare rheological additives for water/oil invert emulsion drilling fluids. The patent gives examples of the clay being used together with fully hydrogenated castor oil, i.e. castor wax.